1. Field of the Invention
The present invention relates to a switching power supply apparatus for supplying and interrupting power from an input power supply and converting the power by using an inductor so as to output a predetermined DC voltage.
2. Description of the Related Art
U.S. Pat. No. 5,736,842 (Patent Document 1) and International Publication No. 95/22092 pamphlet (Patent Document 2) disclose switching power supply apparatuses for reducing loss, in which a switching element is switched when the applied voltage thereto is almost zero.
FIG. 11 is a circuit diagram of the switching power supply apparatus according to Patent Document 1. In this apparatus, a series circuit (clamping circuit) including a switching element S1 and a capacitor CC is connected to a junction point of an inductor LS and a switching element S and is connected to a junction point of a diode D and a smoothing capacitor CF. Also, the clamping circuit is connected in parallel to a series circuit including the inductor LS and the diode D.
In the switching power supply apparatus shown in FIG. 11, energy is accumulated in the inductor LS during an ON period, when the switching element S is conducting, the capacitor CC is charged during an OFF period of the switching element S, and then discharge is performed. By turning off the switching element S1 during the discharge period, electric charge accumulated across the parasitic capacitance of the switching element S is discharged, so that the switching element is turned on such that the voltage across the switching element is zero (hereinafter, this operation is referred to as zero-voltage switching).
FIG. 12 is a circuit diagram of the switching power supply apparatus according to Patent Document 2. In this circuit, an inductor L2 is connected between a junction point of the inductor L2 and a switching element S1 and a fifth diode D5. With this configuration, a series circuit (clamping circuit) including a switching element S2 and a capacitor C3 is connected across the inductor L2 and energy is accumulated in the inductor L2 during an OFF period of the switching element S1, so as to perform zero-voltage switching of the switching element S1.
In the switching power supply apparatus shown in FIG. 11 according to Patent Document 1, commutating energy accumulated in the inductor LS changes when an input voltage and an output current significantly vary, and thus zero-voltage switching is difficult to perform in a wide range of input voltage and load current. In particular, in a switching power supply apparatus having a harmonic current regulating function, switching is performed without rectifying/smoothing a DC input power supply voltage, and thus the input voltage significantly varies. For example, when a commercial AC power supply voltage is AC 264 V, the input voltage changes in a wide range of 0 to 373 V at the commercial AC power supply frequency. Therefore, loss is significantly increased when zero-voltage switching is not performed reliably.
Also, a peak current and an effective current flowing through the inductor LS significantly change with a change in the input power supply voltage. Therefore, it is difficult to accumulate a commutating energy required for zero-voltage switching while decreasing conduction loss by the inductor LS.
Further, when the junction point of the inductor LS and the switching element S and a stable potential (for example, potential across the smoothing capacitor CF) are connected by the clamping circuit, current flows through the inductor L continuously under a light-load condition, so that a current regenerative mode occurs, where current is regenerated in the input. Accordingly, conduction loss by the inductor L and the switching element S increases and efficiency under a light-load condition is significantly deteriorated.
In the switching power supply apparatus shown in FIG. 12 according to Patent Document 2, when an input power supply voltage and an output current to a load significantly vary, commutating energy accumulated in the inductor L2 changes. Therefore, it is difficult to perform zero-voltage switching in a wide range of input power supply voltage and output current. Also, it is difficult to accumulate a commutating energy required for zero-voltage switching while reducing conduction loss by the inductor L2.